This invention relates to a process for the gasification of coal in situ, and is particularly concerned with a novel process for in situ gasification of coal which utilizes certain mining operations or techniques in combination with in situ gasification, to obtain superior results in the production of a product gas having higher or improved BTU content as compared to prior art processes.
The gasification of coal in place relates to the recovery of the energy of the coal without mining. The products are obtained in a gaseous form and may be utilized for producing electric power, the manufacture of organic chemicals and the synthesis of liquid or gaseous fuels. An advantage of the gasification of coal in place is the elimination of complex and expensive underground mining operations and the utilization of coal from beds which are not profitable to mine.
Considerable effort has been expended on studying the in situ gasification of coal, since many coal seams are so thin or of such low grade, or potentially could be exploited most economically by in situ gasification. In "Chemistry of Coal Utilization," pages 1023 and 1040, edited by H. H. Lowry, Wiley and Sons, New York 1963, there is set forth a review of the various processes which have been proposed for the underground gasification of coal. Thus, for example, there is disclosed processes in which a bore hole is driven down into the coal strata, the coal ignited, and air and water passed down, producing a gas containing hydrogen and carbon monoxide. U.S. Pat. No. 3,298,434 discloses a process for in situ gasification of coal wherein a well is first sunk to the coal seam and air and steam are injected and the product gas is recovered from the same injection well. In another form of the process of the above patent, both injection and production wells are drilled into the coal seam and a channel of communication is provided between the injection and the production wells to permit the flow of gases between the wells and the recovery of product gas from the production well.
However, in many of the proposed processes as exemplified by those of the above-noted prior art, the total coal recovery and the BTU content of the gas have been relatively poor because of inadvertent mixing with nitrogen or other inerts and by-passing of coal in the formation. Hence, in large measure, the processes heretofore proposed have been relatively inefficient in producing high yields of a sufficiently high BTU product gas.
Accordingly, one object of the invention is the provision of an improved method of burning and gasifying coal in situ under controlled conditions, to obtain the product gases and product oils.
Another object is to provide a method of gasification of coal which employs a combination of mining and in situ gasification techniques, to obtain a product gas of relatively high BTU content.
A still further object is the provision of an improved method of in situ gasification of coal under controlled conditions by injection of air and steam, in conjunction with certain mining techniques, so as to separate flue gases from the product gas and obtain high yields and a high quality product gas.